The present invention relates to ammonium nitrate fuel oil mixtures.
Ammonium nitrate-containing explosives are manufactured and used widely in large volumes. Ammonium nitrate is a relatively strong oxidizing agent. However, it is not readily detonated by itself, and it is therefore generally admixed with various fuels, modifiers and sensitizers which themselves are either explosive or non-explosive. These ammonium nitrate-containing explosives may be divided into four general types: dry blasting agents; slurry or gel explosives; emulsion (and emulsion blend) explosives; and nitroglycerin-based explosives.
For many years, ammonium nitrate fuel oil (“ANFO”) has been one of the most popular explosives for use in mining operations. ANFO compositions are formed of porous ammonium nitrate prills and diesel oil that is situated within many of the voids of the porous ammonium nitrate prills. In mining operations, the use of ANFO typically involves drilling a blast hole in the earth that is being mined. After a blast hole has been drilled, one or more initiators and ANFO are loaded into the blast hole. The ANFO is then detonated with the result being that the earth that is being mined is fractured in a manner that facilitates the removal of the earth by machinery for further processing. Typically, an array of blast holes is established and the ANFO established in the blast holes is detonated simultaneously or in a sequence that is designed to produce desired blast characteristics.
In their simplest form ANFO explosives consist of two readily available commercial components: ammonium nitrate (AN) and an organic liquid fuel such as fuel oil or diesel. As mentioned above, the AN is typically used in the form of prills (spherical or roughly spherical particles up to a few mm in diameter, for example from 0.9 to 3.0 mm) and these may be manufactured in such a way, and/or with included minor added components, to improve their porosity giving porous prilled AN (PPAN). The AN and liquid fuel are mixed at specific ratios in order to control the oxygen balance and generate the most efficient explosion possible. Typically the weight ratio used is at or around 94:6 AN:liquid fuel. Regardless of what ratio is desired, it is therefore preferable for the AN, or typically PPAN, to have sufficient porosity to absorb, and retain, the desired amount of fuel. When the ratio of 94:6 is used, it would be desirable for the AN to absorb at least 6% by weight of liquid fuel and then retain this liquid fuel during storage and use.
Good quality PPAN will generally be characterized by the presence of small pores in the prill interior which will confer high oil absorption and good liquid fuel retention (the fuel is absorbed and stays absorbed over time). Low quality PPAN may contain large voids or holes which confer good oil absorption but poor liquid fuel retention (the fuel is absorbed but leaks out over time). Dense AN prills may also be encountered which may have little or no porous nature, or a single large hole or void connected to the exterior; the remainder of the particle being highly crystalline and relatively free of voids. Such dense prills may be expected to show poor absorption and/or retention of the liquid fuel when used to prepare ANFO.
Poor retention of the liquid fuel in the ANFO can cause problems for users and in extreme cases some of the liquid fuel may separate and e.g. may collect in the bottom of bags or other containers of ANFO. If such separation occurs either before or after an ANFO product is loaded into a borehole a poor blast may result from an incorrect ratio of AN and liquid fuel (incorrect oxygen balance).
Thus there is a need to improve the fuel retention properties of ANFO compositions, particularly with regards to low quality PPAN.
Furthermore, a major disadvantage of ANFO as a blasting product is that AN is very soluble in water. ANFO has little or no resistance to water and therefore cannot always be used in wet conditions. Different systems have been proposed to improve on the water resistance of ANFO by the addition of various additives such as gelling agents, hydrophobic thickeners and hydrophobic compounds. These additives can work to repel water and/or form a barrier between water and the bulk of the ANFO product. The use of these additives adds cost to the compositions and low quality PPAN is even more susceptible to water.
Thus there is a continued need to improve the water resistance properties of ANFO compositions, particularly with regards to low quality PPAN.